Please refer to FIG. 1 which is a schematic diagram showing a topologic structure of a conventional uninterrupted power supply. It is obvious that if the load in such a topologic structure is required to remain stable when the input power IP1 is interrupted or unstable, the conventional uninterrupted power supply has to include an AC/DC (alternating current/direct current) converter 1 rectifying the alternating input power signal IP1 to generate a direct current signal IP2, a DC/DC converter 2 converting a direct current signal IP2' into a higher level direct current signal IP3, an inverter 3 convening the direct current signal IP2 or IP3 into an alternating current signal IP4, and a power reservoir 4 providing reserve power for the load 5, which are electrically interconnected as shown in FIG. 1.
Another conventional uninterrupted power supply is shown in FIG. 2. This type of uninterrupted power supply also includes an AC/DC converter 1, a DC/DC converter 2, a DC/AC inverter 3 and a power reservoir 4, which have similar functions as those shown in FIG. 1. However, the AC/DC converter 1 further includes a rectifying circuit 11 and a power factor correction (PFC) circuit 12. The direct output RP1 of the power reservoir 4 is arranged into the PFC circuit 12, and the PFC circuit 12 is controlled by a PFC control circuit 13 which includes a PFC IC. The output signal IP2' of the PFC circuit 12 is transmitted to the DC/DC converter 2 to be converted into a higher level direct current signal. Then, a stable and uninterrupted power signal can be provided for the load 5 through the inverter 3.
In order to further illustrate the typical circuit structures inside the blocks of FIG. 2, a schematic circuit structure showing the converter 1 including the rectifying circuit 11 and the PFC circuit 12, and the DC/DC converter 2 is provided, as shown in FIG. 3. The working principle of the circuit structure is well known to those skilled in the art, and is not to be redundantly described here.
The conventional uninterrupted power supplies are generally categorized to possess the aforementioned topologic structures, and have disadvantages as follows.
1. Owing to the existence of the electrically isolated DC/DC converter 2, the conventional uninterrupted power supplies have adverse size and weight.
2. In conventional power supplies, the input power signal IP1 has to be transmitted through several converters 1, 2 and 3 for multi-stage transformation before it is applied to the load 5, so the overall efficiency is reduced due to the frequent conversion operations.
3. The existence of the DC/DC converter 2 makes the conventional power supplies more complicated in structure so that higher cost and lower reliability are caused.